After years of steady decline, only 25 polio cases were reported in Afghanistan in 2010, prompting one international health care official to declare that, “the Afghans are heroes.’’ Then last year, the number tripled to 76, the Afghan Ministry of Public Health said. While the total remains small, polio is highly contagious and health experts say that each detected case is an indicator of hundreds of “silent’’ ones, mainly children with mild infections who become carriers.